1. Field of the Invention
The present invention relates to a lighting device manufactured using a light-emitting element in which a light-emitting layer containing an organic compound is provided between a pair of electrodes.
2. Description of the Related Art
In recent years, a light-emitting element (also referred to as an electroluminescent (EL) element) in which a light-emitting layer (hereinafter also referred to as an EL layer) containing an organic compound is interposed between a pair of electrodes has been actively developed. Lighting has attracted attention as one of application fields of the light-emitting element. This is because a lighting device including an EL element has features different from those of other lighting devices; for example, the lighting device including the EL element can be thin and lightweight, and perform surface emission.
In addition, an EL element has attracted attention also in terms of its high efficiency of conversion from power into light and its high potential for saving energy. It is also a unique feature of an EL element that, depending on a substrate selected, it is possible to provide a lighting device having flexibility, a lighting device having high resistance against an impact such as physical destruction, or a very lightweight lighting device.
Unfortunately, a lighting device including an EL element has a significant problem in its life. This is partly because an EL element is significantly deteriorated by factors which accelerate deterioration, such as moisture and oxygen. Further, a lighting device which performs surface emission continuously in a large area has a problem of heat generation even when the lighting device includes an EL element with high power conversion efficiency. Such a temperature increase accelerates deterioration.
When flexibility, impact resistance, and weight reduction are demanded, these problems become more severe. Generally, a glass substrate is used for formation of an EL element. Having low impact resistance, little flexibility, and heavy weight, a glass substrate also has a superior gas barrier property and thus can effectively block factors for deterioration, such as water and oxygen in an outside atmosphere, which makes it possible to manufacture a long-life EL lighting device. In contrast, a resin substrate used for adding flexibility, increasing impact resistance, and reducing weight has an inferior gas barrier property; thus, an EL element is deteriorated rapidly as compared to the case of using a glass substrate.
Also proposed is an organic light-emitting display formed using a metal substrate as a substrate having both favorable sealing performance and a countermeasure against heat generation (for example, see Patent Document 1).